In the rubber industry there is a general desire for improved processability of the rubbers that are used, especially as regards the flow characteristics. The lower the viscosity of the rubber, the simpler the processing technology, the greater the productivity, and thus the less wastage there is. These aspects are very important, especially with fluororubbers, since these are expensive rubbers that cannot be processed completely on injection moulding machines used in the rubber industry.
Thanks to its high throughput and simplicity, the liquid injection moulding technique has found wide use in the elastomer domain, in particular for the processing of silicon rubbers, typically provided as two-components liquid mixtures, comprising a crosslinkable resin and a catalyst.
So-called ‘liquid’ or low viscosity fluoroelastomers have been proposed in the art as materials intended for processing notably via liquid injection moulding machines, i.e. processing tools wherein much lower temperatures and pressures are used with respect to traditional injection moulding devices which are intended to process molten materials.
Traditional injection moulding machines generally consist of a material hopper, an injection ram or screw-type plunger, and a heating unit or press, which holds the mould in which the components are shaped; in the liquid injection moulding devices, the ram or plunger is merely replaced by a metering injectors. For sure, pressure applied both for forcing flow of the material in the press and for achieving precise shaping in the press itself are extremely different in these alternative configurations, a standard injection moulding device working typically with molten polymers at pressures of 100÷200 bar, while in a so called ‘liquid’ injection moulding machines, pressures of 15÷20 bar being largely sufficient to promote flow of material into the mould.
Further in addition, liquid status or low viscosity are required for processing according to screen printing or form-in-place techniques. According to the former technique, a woven mesh is generally used for supporting a stencil. Said stencil thus provides open areas enabling transferring a liquid precursor of the elastomer ink (either neat or as a solution) as a sharp-edged image onto a substrate. To this aim, a roller or squeegee is generally moved across the screen stencil, forcing or pumping the liquid ink past the threads of the woven mesh in the open areas. Subsequent curing and optionally evaporating/drying the solvent carrier enables fixing the elastomer into target shape. Within this processing technique, the use of liquid or low viscosity elastomers could enable avoiding the use of liquid carriers, with substantial advantages in equipment simplicity, environmental friendliness and processing speed.
As per the latter technique is concerned, the rubber precursor is dispensed onto the parts in a viscous uncured form using dispense nozzles especially designed, e.g. to attain specific gasket sizes & shapes. Viscosity of the elastomer should be such to be efficiently ejected from nozzles, while still retaining the shape of ejected form onto the part until curing occurs. The dispensed parts can either be cured using an inline heating oven or placed in a stand alone oven for batch curing.
The challenge of the use of these techniques for processing fluororubbers is to simultaneously achieve a suitable low liquid viscosity of the fluoroelastomer, to advantageously ensure appropriate flow of the material in the mould, during the roll coating and/or in the dispenser nozzle, while still ensuring outstanding curing capabilities, so as to provide for finished article having required mechanical and sealing properties at reasonable throughput rate.
Liquid fluoroelastomers endowed with reduced viscosities have been disclosed in the art.
Thus, document U.S. Pat. No. 5,852,125 (BAYER AG) 22 Dec. 1995 discloses VDF-based fluoroelastomers which can be pumped in liquid state at low temperatures (60-120° C.) and which are readily crosslinkable, said fluoroelastomers having a molecular weight of 3 000 to 30 000, comprising iodine and/or bromine as cure-site and comprising fluorine containing monomers, among which mention is made of vinyl ethers of formula CF2═CF—O—X, with X being a perfluoroalkyl C1-C3 or a group of formula —(CF2CFYO)n—Rf, with n=1-4, Y═F or CF3 and Rf being a C1-C3 perfluoroalkyl.
Similar materials are also disclosed in document WO 98/15583 (BAYER AG [DE]) 16 Apr. 1998, which pertains to liquid fluoroelastomers having iodine content of 0.5 to 2.5% wt; having Mn between 10 000 and 25 000, with a polydispersity index (Mw/Mn) of strictly less than 1; said fluoroelastomers being VDF-based polymers further comprising fluorine containing monomers, among which mention is made of vinyl ethers of formula CF2═CF—O—X, with X being a perfluoroalkyl C1-C3 or a group of formula —(CF2CFYO)n—Rf, with n=1-4, Y═F or CF3 and Rf being a C1-C3 perfluoroalkyl.
Also, TFE-containing liquid fluoroelastomers have been disclosed; thus, document US 2003166807 (BAYER AKTIENGESELLSCHAFT) 4 Sep. 2003 pertains to liquid fluororubbers comprising recurring units derived from TFE (in an amount exceeding 10% moles), said fluoroelastomers:                containing up to 1% wt of iodine or bromine;        having a number averaged molecular weight of more than 25 000;        further comprising recurring units derived from comprising fluorine containing monomers, among which mention is made of perfluorinated vinyl ethers of formula CF2═CF—O—X, with X being a perfluoroalkyl C1-C3 or a group of formula —(CF2CFYO)n—Rf, with n=1-4, Y═F or CF3 and Rf being a C1-C3 perfluoroalkyl.        
Nevertheless, experiences with fluoroelastomers as above detailed have shown that even selecting fluoroelastomers having limited molecular weight (e.g. a weight averaged molecular weight of about 10÷30 k), the viscosity of the liquid still might not be optimized for processing these materials according to liquid injection moulding, screen printing of form-in-place techniques.
Incidentally, EP 1148072 A (AUSIMONT SPA) 24 Dec. 2001 discloses fluoro-vinylethers of formula CFX′═CXOCF2OR and polymers obtained therefrom, either by polymerizing said monomers alone or by polymerizing the same in combination with other (per)fluoromonomers. No general teaching is provided in the specification as per the molecular weight of such polymers. Among exemplified working embodiments, Ex. 7 pertains to a CF2═CFOCF2O—CF2CF3) homopolymer, which is taught as possessing a molecular weight as determined by 19F-NMR of 50 000. Similarly, Ex. 8 discloses a copolymer obtained by co-polymerization of CF2═CFOCF2O—CF2CF2OCF3 and CF2═CFOCF2O—CF(CF3)OCF3, which is taught as possessing a molecular weight as determined by 19F-NMR of 35 000.
There is thus still a current shortfall in the art for liquid fluoroelastomers that can at least be pumped at limited temperatures and that can be processed not only in conventional processing machinery but also in liquid injection moulding, screen printing and form-in-place devices maintaining outstanding cross-linking behaviour, and yielding resulting final parts exhibiting good mechanical as well as ageing properties, substantially similar to those of conventional solid fluororubbers.
The object of the present invention is therefore to provide fluoroelastomers that advantageously exhibit this property profile.